Northern Portugal and Galicia are a special case, seeing as the social stratification originated from "prestigious" megalithic constructions, that originated a species of ancestor and heroic worship, and the descendants of these ancestors were regarded as more important in the community, which in turn lead them to leadership positions. Also, the terrain heavily influenced the sites that were chosen for settlement, and although this elevated and walled enclosures were easily defensible, they were a pain to access and to sustain commercial ties with far-off peoples, which in turn slowed down contacts and cultural exchange.
Regarding the second point, I would argue that the motor for socio-settlement structural change was the fact that contact with oriental cultures (Hellenes, Phoenicians), that had much more complex societies and architectural constructions, were spread across the Mediterranean Basin. It is easy to then infer that these contacts and ideas would take much longer to reach the Netherlands and Jutland, seeing as their main "envoys", the Romans and Southern Celts, confined themselves to the Left Bank of the Rhine, and due to military conflict, it took much longer for these "innovations" to permeate those regions.
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